Centrifugal compressors are used in turbochargers, superchargers, and the like. They comprise a centrifugal compressor wheel that includes an array of aerodynamically contoured impeller blades supported by a central hub section. The hub is mounted on a rotatable driven shaft that is driven, in the case of a turbocharger, by the turbine wheel. For turbochargers, the hub section generally includes a central axial bore into which the shaft extends and is fastened to the hub. Fastening can take any suitable form, such as the use of a threaded shaft and hub, a keyed hub or, alternately, a nose of the shaft may extend through the hub and be fastened thereto using a nut to tighten the hub against a shoulder or other diametrically enlarged structure rotatable with the shaft. The shaft thereby rotatably drives the centrifugal compressor wheel in a direction such that the contoured blades axially draw in air and discharge that air radially outwardly at an elevated pressure level into a chamber of a compressor housing. The pressurized air is then supplied from the chamber to the air intake manifold of an internal combustion engine for admixture and combustion with fuel, all in a well-known manner.
Improvements in compressor technology and design have resulted in increased compressor efficiencies, flow ranges and rapid transient response by careful design of the compressors, particularly the centrifugal compressor wheels. For example, the impeller blades include compound and highly complex curvatures designed to optimize operational efficiency and flow range. The complex blade shapes are generally formed by casting a lightweight metal alloy, including various aluminum alloys, chosen for their relatively low density, to lower the rotational inertia and provide rapid response during transient operating conditions.
While effective, cast centrifugal compressor wheels of this type are subject to metal fatigue that limits the operating lifetime of the turbocharger. For example, a centrifugal compressor wheel may be rotated at operating speeds up to about 100,000 rpm or more. This leads to relatively high radial tensile loading; particularly in the hub portion of the wheel which must support the radial wheel mass. This radial tensile loading is also cyclic in nature during the startup and operation of the internal combustion engine, and the vehicle in the case of a mobile application, into which the turbocharger is incorporated. As the hub is cyclically stressed, inclusions, voids and other defects associated with the casting process provide stress risers resulting in fatigue processes that limit the operational life of the wheels and turbochargers that incorporate them. The use of forged or wrought materials to improve the operational lifetimes of the alloys is possible, but has generally not been sufficiently economical due to the cost of the machining required to form the complex shapes associates with the hub and blades.
Accordingly, centrifugal compressor wheels that provide the required performance characteristics, including high strength and low rotational inertia, as well as reduced susceptibility to fatigue processes compared to cast wheels are very desirable.